COLUMN TOP RESTRAIN - CsI SAFE

[rummaan17]

Dear All,
I have a concern that I would like to address.

While exporting slab to SAFE for design. SAFE only account for column below with restrains but not column on the top. Though, it consider joint stiffness with col_stiff. I have went through few discussion that favor modeling column on top also as half or full length with fixed or pin support.

I would like to have input from respected engineers regarding their preferable method of practice and its effect. Thanks in advance.

 

[KootK]

I believe that the stiffness of the upper column should be represented in your model. Otherwise, you're bound to underestimate negative flexural and punching shear demands which can cause problems. Some options for doing that:

1) Add the upper columns and model the columns above and below to their true heights and fixed at the far ends. The code procedures would seem to suggest that this is appropriate.

2) Add the upper columns and model the columns above and below to half of their true heights but pinned at the far ends. Some prefer this as, in a tall repetitive structure, your column inflection points are likely to be at approximately half of the story heights.

3) Don't add the upper columns but fiddle with the stiffness of the lower columns so that the effective stiffness is the same as option one or option two. I prefer to just model the upper columns. It takes no time at all to add them and makes things much more transparent for the next person who opens up the model. Another problem with this strategy is that your unbalanced column moments, as reported by the software, will not be accurate as they will not be divided between the upper and lower columns.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rummaan17]

Dear KootK,

Col_stiff which SAFE add in the model as slab account for the joint stiffness, wouldn't it be sufficient for negative flexure and punching shear?

Second, I read your previous post too. Can you please dictate any code clause that direct towards modeling of Top column ?

Lastly, If I model column with fixed support, should the translation along Z axis be released ?

Awaiting your response. Thanks

 

[KootK]

Col_stiff which SAFE add in the model as slab account for the joint stiffness, wouldn't it be sufficient for negative flexure and punching shear?

I've no idea what "col_stiff" is. At the end of the day, you need a) know how column stiffness is accounted for in the software and b) ensure that you've accurately modeled the stiffness of both the columns below and the columns above.

Can you please dictate any code clause that direct towards modeling of Top column ?

I know of no such code clause. But, then, do you really need a code clause to tell you that your structural model should accurately reflect structural reality? I think not.

Lastly, If I model column with fixed support, should the translation along Z axis be released ?

I don't think that it matters for slab design. Given the choice, I'd fix Z translation at the bottom and leave Z translation free at the top.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rummaan17]

Dear KootK,

I know of no such code clause. But, then, do you really need a code clause to tell you that your structural model should accurately reflect structural reality? I think not.

This is the problem I just faced when i was designing slab on grade separately and I modeled column above with fixed support. The doctor in the peer panel insisted me to remove it as it doesn't make sense to him.

As you also quoted in your second last reply point-1 that code also point towards it. Sometime when engineering though differ, a line from Code always seems to be reasonable thing to mention. I hope you understand why I asked you for it.

 

[KootK]

This is the problem I just faced when i was designing slab on grade separately and I modeled column above with fixed support. The doctor in the peer panel insisted me to remove it as it doesn't make sense to him.

That makes sense as a slab on grade is not normally constructed so as to have flexural continuity with the columns above.

As you also quoted in your second last reply point-1 that code also point towards it. Sometime when engineering though differ, a line from Code always seems to be reasonable thing to mention.

The section of ACI that describes the equivalent frame method mentions the columns being fixed at their far ends. That's all that I know of.

. I hope you understand why I asked you for it.

I understand but do not agree. If a reviewer asked me why I modeled an upper level column in the design of a suspended slab, I would simply respond "because there is a flexurally continuous upper level column attached to the slab". I don't need a code clause to tell me that my dog is a dog and not a cat.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rummaan17]

Thanks alot @kootK

 

[KootK]

You're welcome rummaan. Despite the reviewer hassles, it sounds as though it's quite an interesting project.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rummaan17]

Mr. Kootk Indeed I am learning alot from this PeerReview. Also, I am very obliged to all professional like you who spare time to help new learner like me. I owe alot to our engineering community and that what I have started realizing now. Thanks again for everything.